I was talking earlier to a script kiddie and claimed that IP spoofing can increase the upload speed in a DDOS attack?! that does not make any sense to me According to him if your server has an upload speed of 1Gbps IP Spoofing will help you strike a target with a 10Gbps DDOS attack, any explanation? Thank you for your answers
He is referring to an attack such as a DNS Amplification Attack. He is not increasing his upstream, but he would be increasing the amount of data being sent to the target.
To make it simple:
You can find more details on the details of DNS amplification attacks here.
There are other services that are vulnerable to these types of attack too, including NTP. It also might be worth noting that in my experience, it is very rare that you will find a residential ISP that will forward spoofed packets.